1. Field of the Invention
The present invention relates to a high efficiency power amplifying apparatus having a class D power amplifier, and more particularly relates to a low noise, high efficiency power amplifying apparatus suitable for audio signal amplification.
2. Description of the Related Art
High efficiency power amplifiers according to the related art that are used to drive loudspeakers typically use some type of class D power amplifier for power amplification because of their high power efficiency and suitability for audio signal amplification. A high efficiency power amplifying apparatus using this type of class D power amplifier is therefore described below with reference to the accompanying FIG. 19.
Shown in FIG. 19 are: a signal input terminal 301, class D power amplifier 302, speaker input terminal 303, class D power amplifier output terminal 304, positive supply terminal 305 and negative supply terminal 306 for the class D power amplifier 302, passive low-pass filter 307, and speaker 326. The passive low-pass filter 307 further comprises coils 310A, 310B, and 310C, and capacitors 311A, 311B, and 311C.
A high efficiency power amplifying apparatus thus comprised according to the related art operates as described below.
An audio signal input to the signal input terminal 301 is first power amplified by the class D power amplifier 302 using pulse width modulation (PWM) with a switching operation. A heavily pulse-width modulated carrier signal is therefore output from the output terminal 304 of the class D power amplifier 302. To remove this modulated carrier signal, the passive low-pass filter 307 operates as a sharp six-stage Butterworth type filter having three coil and capacitor stages.
A typical filter characteristic is shown in FIG. 20. As will be known from FIG. 16, the cut-off frequency (fc) of the filter is normally set somewhere above 20 kHz, that is, above the highest audio signal frequency, so that the audio signal is passed to the speaker without attenuation. If the PWM carrier frequency is 200 kHz, for example, attenuation, which is determined by the characteristics of the low-pass filter, is limited (-60 dB, for example). The PWM carrier frequency component at the speaker input terminal 303 is thus input to the speaker 326 as residual noise.
With a high efficiency power amplifying apparatus according to the related art as described above, a limited amount of residual noise from the PWM carrier frequency component is superimposed on the audio signal no matter how sharp the filter characteristics of the passive low-pass filter 307. This noise component is emitted from the speaker wire as high frequency noise that can adversely affect all kinds of electronics. Expensive coils and capacitors are also needed to build a high order passive low-pass filter, and high costs must therefore be incurred to achieve an anti-noise filter.
With consideration for the above-noted problems, an object of the present invention is therefore to provide a high efficiency power amplifying apparatus which overall is less expensive and achieves less high frequency noise than a high efficiency power amplifying apparatus according to the related art while also minimizing the use of expensive passive filters.